As is well known and understood, one of the major concerns of designers of antenna system communications links is the elimination or reduction of external interference sources such as jamming, self-interference, atmospheric noise and man made noise. As is well known, most arrangements which attempt to resolve these problems of external interference do so in a relatively complex manner, often utilizing very large directional antennas and/or with antennas having hundreds or more elements. This problem of external interference is particularly prevalent in the area of mobile communications systems where omnidirectional antennas are employed because of the large number of users operating in the same frequency band and because of multipath. Use of very large directional antennas in mobile communications, moreover, is almost physically impossible and economically impractical.
A new concept for eliminating interference in a communications system utilizing a plurality of transmission links is shown and described in U.S. Pat. No. 4,275,397, entitled, "Interference Cancelling Random Access Discrete Address Multiple Access System", which issued to Frank S. Gutleber, the present inventor, on June 23, 1981. The system disclosed therein utilizes orthogonal multiplexing in conjunction with a receiver antenna configuration comprised of an omnidirectional antenna and a notch antenna at a receiving end of the transmission link to cancel interference arriving from all directions except over the narrow beamwidth notch or null formed by the notch antenna.
In the above referenced related application, there is shown and disclosed a transmitter for use in mobile subscriber multiple access system where the power output of the communications transmitter is fed to both an omnidirectional antenna and a notch antenna having a single null over a small angular beamwidth with the combination developing a relatively narrow beam of radiation at the location of the null. By rotating the notched antenna or electronically scanning it, the notch and accordingly the narrow beam of radiation can be pointed in the direction of the desired signal.
Accordingly, it is an object of the present invention to improve the angular orientation of an antenna system at the receiving end of a transmission link to eliminate external interference which thereby provides a high degree of angular resolution and facilitates intercept.
Another object of the invention is to provide an accurate measurement of the angular bearing of received radiation from a plurality of discrete radiating sources.
Still a further object of the invention is to provide a means of achieving exceptionally fine angular resolution of antenna direction toward one or more radiating sources in a dense communications environment.
And yet another object of the invention is to provide accurate angular direction finding (D.F.) of received radiation from a large quantity of radiating sources by sweeping a received radiation pattern having a narrow beamwidth portion which sequentially isolates each radiating source located in a dense communications environment.